Ferrite-based sintered magnets are used as magnets for, for example, magnet rolls in AV and OA devices and copying machines that are required to have strong magnetic force.
However, such ferrite-based sintered magnets have their specific problems in that cracking and chipping can occur, that their productivity is low since polishing is required, and that processing into a complicated shape is difficult.
In recent years, bonded magnets using rare-earth magnets are used in part of this field.
However, the problems of the rare-earth magnets are that they are about 20 times more expensive than the ferrite-based sintered magnets and rust easily.
Accordingly, there is a demand to replace the ferrite-based sintered magnets with ferrite-based bonded magnets.
However, the maximum energy product BHmax of bonded magnets is lower than those of sintered magnets. Therefore, bonded magnets used as alternatives must be further improved to have higher maximum energy product BHmax.
To improve the maximum energy product BHmax, residual magnetic flux density Br and coercive force Hc must be improved. To improve the former property, it is important to improve the saturation magnetization value as, filling properties, and orientation properties of ferrite powder. To improve the latter property, it is important to improve crystallinity and to suppress the formation of multi-axis ferrite particles.
Several methods proposed to improve the saturation magnetization value as include the use of ferrite powder having a W-type crystal structure or the addition of an additional element (such as a rare-earth element or a cobalt element) to crystals to form a solid solution (Patent Literature 1). However, the range of improvement is limited to several percent. In addition, the production methods are complicated, and an expensive additional element is used, leading to an increase in cost whereas unfortunately, the advantages obtained are not so great. Therefore, the resulting magnets are not enough to replace ferrite-based sintered magnets.
One method proposed to improve the filling properties is to mix two or more types of ferrite powders with different particle diameters so that small particles enter the spaces between large particles. The filling factor is thereby improved (Patent Literature 2).
However, the ferrite powders to be mixed contain a large amount of hexagonal plate-like particles. This is disadvantageous to ensure both the filling properties and flowability of the ferrite powders in the compound, and no sufficient consideration is given to the dispersibility of the ferrite particles.
The flowability greatly affects the kneadability and moldability of the compound, and, in extreme cases, the compound cannot be kneaded or molded.
The flowability affects the final orientation of the ferrite particles in a molded product. Therefore, the reduction in the orientation properties caused by increasing the filling amounts of the ferrite powders must be compensated, and a high orientation magnetic field of 10 kOe or more must be used during molding. The use of a large-scale molding apparatus can, of course, result in an increase in production cost, and therefore this method is also not enough to replace ferrite-based sintered magnets.
The orientation properties are greatly affected by the flowability of a compound, as described above.
The flowability is also greatly affected by the resin and surface treatment agent used for the compound. However, it is advantageous for the ferrite powder to include a small amount of particles having a hexagonal plate-like shape, which is a typical shape of a highly crystalline ferrite powder, and for the particles of the ferrite powder to have high dispersibility and a small specific surface area (a large particle size).
However, if the particle size is large, magnetic domain walls are easily formed to form multi-axis particles, and this results in a reduction in coercive force, so that the particle size cannot be simply increased.
To improve the coercive force, it is advantageous to improve crystallinity and reduce the particle size so that the formation of multi-axis particles can be suppressed.
However, if annealing temperature is increased to improve the crystallinity, aggregation (sintering) may proceed to reduce the dispersibility.
If the particle size is reduced, the flowability is lowered. Therefore, it is difficult to improve the coercive force Hc while the residual magnetic flux density Br is maintained.